Surface wave filter for asymmetrical/symmetrical and symmetrical/symmetrical operating mode

ABSTRACT

Surface wave filter for asymmetrical/symmetrical and symmetrical/symmetrical operating mode, with interdigital output transformers (IDT2, IDT4) connected in series.

BACKGROUND OF THE INVENTION

The invention relates to a surface wave filter, hereafter referred to asOFW, which can be used in particular for RF applications.

In communication systems, signals are processed asymmetrically orsymmetrically, whereby changes between the two operating modes oftentake place in the routing of the signal, and so a need arises forfilters that, regarded at the input side or at the output side, can beoperated asymmetrically/symmetrically or symmetrically/symmetrically. Inaddition, certain circuit designs require filters with thecharacteristics stated above, and in addition with different input andoutput impedances.

OFW filters that can be operated asymmetrically/asymmetrically, intendedin particular for use in the RF range, are commercially available. Thechange from asymmetrical to symmetrical operating mode has up to nowstandardly been carried out by means of additional components, e.g.,transformers. The impedances, i.e., the input and output impedances ofthese filters, are equal, and are matched by means of networksconsisting of inductances and capacitances.

OFW filters that can be operated asymmetrically/symmetrically orsymmetrically/symmetrically are also known. FIG. 1 shows a schematicrepresentation of the design of such a filter, i.e., in the present caseof a single-track filter with identical input and output impedances.

The structure of this OFW filter consists of an interdigital inputtransformer IDT3, centrically arranged, and of the reflectors REF1 andREF5, short-circuited in themselves, and interdigital outputtransformers IDT2 and IDT4 connected in parallel.

If the filter selectivity is to be increased, then, as FIG. 2 in turnschematically shows (identical elements are thereby designated withidentical reference characters), the known single-track filter accordingto FIG. 1 can for example be connected in series with a filter of thesame type. This filter also has identical input and output impedancesand can be operated asymmetrically/symmetrically orsymmetrically/symmetrically.

SUMMARY OF THE INVENTION

As already mentioned above, the underlying object of the presentinvention is to create OFW filters that can likewise be operatedasymmetrically/symmetrically, symmetrically/symmetrically, orasymmetrically/asymmetrically, but have different input and outputimpedances.

For the solution of this object, in an OFW filter the invention providesthat the interdigital output transformers are connected in series, andin addition are preferably mirrored to one another about the directionof propagation of the surface waves.

By means of the mirrored arrangement of the output transformers, theirterminals can be applied to one side of the filter structure, and thecorresponding contact paths can thereby be kept short. This contributesto a space-saving filter structure and, if necessary, reduces disturbinginductive and capacitive influences.

If, according to a further proposal of the invention, the above OFWfilter is connected in series with an OFW filter consisting of a basicelement with OFW single-port resonators connected in series and parallelhereto, there results a filter that can be operatedasymmetrically/symmetrically or symmetrically/symmetrically, havingdifferent input and output impedances, which filter unites the highremote selectivity of the single-track filter structure with the highadjacent-channel selectivity and the low insertion loss of the connectedsingle-port resonators.

Independent of the above OFW filter, according to a further inventiveproposal a filter with identical input and output impedance, which beoperated asymmetrically/symmetrically, symmetrically/symmetrically, orasymmetrically/asymmetrically, is created solely by means of thecascaded (if necessary) connection of the above basic element, whichfilter unites high adjacent-channel selectivity with an insertiondamping that is low but that increases with an increasing number ofcascaded basic elements.

BRIEF DESCRIPTION OF THE DRAWINGS

The features of the present invention which are believed to be novel,are set forth with particularity in the appended claims. The invention,together with further objects and advantages, may best be understood byreference to the following description taken in conjunction with theaccompanying drawing, in which:

FIGS. 1 and 2 depict prior art filters;

FIGS. 3, 4 and 5 schematically show a first, second and third embodimentof an OFW filter according to the invention;

FIG. 6 schematically shows a known OFW single-port resonator;

FIG. 7 schematically shows a wiring diagram of a further embodiment ofan OFW filter according to the invention;

FIG. 8 schematically shows the transmission function of the OFW filteraccording to FIG. 7;

FIGS. 9, 10 and 11 schematically show further embodiments of the OFWfilter according to FIGS. 5 and 7.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 3 shows a structure, applied on a known piezoelectric substrate(not shown), of an OFW single-track filter with interdigital inputtransformer IDT3, output transformers IDT2 and IDT4 connected in series,and reflectors REF1 and REF5. By means of mirroring of the outputtransformers to one another about the direction of propagation of thesurface waves, the separate outputs thereof are arranged on one side ofthe filter structure. By this means, long terminal contact paths areomitted, which reduces possible disturbing inductive and/or capacitativeinfluences, and additionally leads to a savings of space.

By means of the series circuit of the two output transformers IDT2 andIDT4 the impedance, as sought, is increased at the output side, and ise.g., 150 Ω at 50 Ω input impedance. As is additionally required, thefilter can in addition be operated asymmetrically/symmetrically orsymmetrically/symmetrically.

An OFW two-track filter that can be operated in the above manner,likewise with different input and output impedance, is shown in FIG. 4,whereby the single-track filter according to FIG. 3, connected in serieswith its output transformers, is connected in series with at least onesingle-track filter with interdigital output transformers connected inparallel (see FIGS. 1 and 2), in order to increase the selectivity ofthe filter.

In the embodiment according to FIG. 5, the OFW single-track filteraccording to FIG. 3 is connected in series with an OFW filter consistinge.g. of a basic element that can among other things be operatedsymmetrically/symmetrically (shown separately in FIG. 7), with OFWsingle-port resonators RES 1a, RES 1b or, respectively, RES 2a, RES 2b,connected in series and parallel hereto (see here also the single-portresonator RES according to FIG. 6 with interdigital transformer IDT andreflectors REF, together with the associated switching diagram).

This filter likewise fulfills all the requirements according to theobject of the invention. It is additionally distinguished in that itunites the high remote selectivity of the OFW single-track filteraccording to FIG. 3 with the high adjacent-channel selectivity and lowinsertion loss of the basic element. Concerning this, see thetransmission characteristic according to FIG. 8 of the basic elementaccording to FIG. 7. By means of the addition of additional basicelements, i.e., by means of a cascaded connection, as shown in FIG. 10,in addition the remote selectivity can be increased with increasinginsertion loss.

For reasons of economy of space, it is additionally desirable to replacethe single-port resonators RES 2a and RES 2b, connected in parallel,respectively with an individually correspondingly dimensionedsingle-port resonator RES 2c, as shown in FIGS. 9 and 11.

The specified basic element, connected in cascaded manner if warranted,is in addition usable separately, i.e. in itself, if an OFW filter isrequired that is to be operated e.g. only symmetrically/symmetrically.By means of the corresponding selection of the single-port resonatorsRES 1a, RES 1b, connected in series, and the single-port resonators RES2a, RES 2b, connected in parallel hereto, there results the transmissioncharacteristic of a band-pass filter, as shown in FIG. 8. The input andoutput impedance of this filter is equal, and is set individually bymeans of simple modification in particular of the aperture and fingernumber of the interdigital transformers.

The invention is not limited to the particular details of the apparatusdepicted and other modifications and applications are contemplated.Certain other changes may be made in the above described apparatuswithout departing from the true spirit and scope of the invention hereininvolved. It is intended, therefore, that the subject matter in theabove depiction shall be interpreted as illustrative and not in alimiting sense

What is claimed is:
 1. A surface wave filter that can be operatedasymmetrically/symmetrically or symmetrically, comprising:at least oneinter-digital input transducer; first and second inter-digital outputtransducers, said first output transducer corresponds to a mirror imageof said second output transducer with a propagation direction of asurface wave as axis of reflection; a further surface wave filterconnected in series with said surface wave filter, whereby the furthersurface wave filter includes a basic element, said further surface wavefilter includes one-gate resonators that are connected in series andone-gate resonators that are connected in parallel.
 2. The surface wavefilter according to claim 1, wherein basic elements are connected incascaded fashion, consisting of surface wave single-port resonatorsconnected in series, with surface wave single-port resonators connectedin parallel, said surface wave filter having substantially identicalinput and output impedances.
 3. The surface wave filter according toclaim 1 wherein said surface wave filter further comprises, acentrically arranged interdigital input transformer with saidinterdigital output transformers and reflectors arranged on both sidesthereof, and said interdigital output transformers are connected inseries.
 4. The surface wave filter according to claim 1, wherein saidsurface wave filter further comprises: at least one single-track filterconnected in series with said surface wave filter, said at least onesingle-track filter having its interdigital output transformersconnected in parallel.
 5. The surface wave filter according to claim 1,wherein a plurality of basic elements are connected in cascaded fashion.